Transcript Electromagnetism

Electromagnetism
BADI Year 2
John Errington MSc
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Coils, Motors, Generators and
Transformers
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History of developments in electromagnetism
• 1800: Volta develops the voltaic pile – the first electrochemical cell
and battery capable of producing continuous electric current.
• 1820: Oersted discovers a current flowing in a conductor causes a
magnetic field.
• 1820: Ampere discovers a force between two wires carrying
currents.
• 1831: Faraday showed electricity could be produced by magnetism.
Sets basis for electric motor and generator.
• 1860 James Clerk Maxwell produces a set of equations that puts the
theory of electromagnetism on a mathematical basis.
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Michael Faraday
• The Prime Minister of the day is said to
have asked him (Faraday) what use could
be made of his discoveries. Faraday
allegedly responded, "Someday it might be
possible to tax them."
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Michael Faraday
• Faraday's first law of electromagnetic induction
– An electromotive force (voltage) is induced in a conductor when
the magnetic field surrounding it changes.
• Faraday's second law of electromagnetic induction
– The magnitude of the electromotive force is proportional to the
rate of change of the field.
• Faraday's third law of electromagnetic induction
– The sense of the induced electromotive force depends on the
direction of the rate of the change of the field.
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Maxwell
•
•
•
•
The first equation is really Faraday's Law of
Induction. It states that an induced electric field (E)
is created by a changing magnetic flux density
(dB/dt) with a polarity that opposes the changing
magnetic field (-). The faster the flux density
changes, the greater the induced electric field.
In the second equation, Oersted and Ampere and
Gauss showed that a current (J) would create a
magnetic field (H). However, Maxwell took it further
and showed that a magnetic field (H) is created by a
current (J) and a changing electric field (dD/dt).
In the third equation, Coulomb and Gauss showed
that an enclosed electrical charge (p) will create a
net electric field (D). In other words, if you were to
enclose an electron within a soap bubble, there
would be a net electric field created by that electron
which is a single negatively charged particle.
The fourth equation, also by Gauss, states that an
enclosed magnet will have a net magnetic flux (B)
of zero. In other words, every magnet has a north
pole and a south pole, so that if you were to enclose
even a part of a magnet within a soap bubble, the
total number of magnetic field lines entering the
bubble would equal the total number of magnetic
field lines exiting the bubble, with a net of
zero. Thus there is no monopole, or particle, which
has just one magnetic pole without the other. This
would be like having a magnet with just a north
pole, but no south pole.
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Conductors carrying current
generate a magnetic field.
A straight wire carrying a current generates
a magnetic field around the wire.
The direction of the magnetic field obeys the
‘corkscrew rule’
Current flow
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Electromagnets
If wire is wrapped around a plastic former to form a coil it
will generate a magnetic field when a current is passed
through it. The magnetic field strength is proportional to
the number of turns, and the current.
If you grasp the coil with your right hand with your fingers
pointed in the direction of current flow your thumb will point
toward the N pole of the coil.
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Magnetic field of a coil of wire
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Magnetic field around
a single loop
Magnetic field in a coil with
an iron core
Magnetic field in
an air cored coil
The size of the air gap has the biggest
influence on the strength of the magnet
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Basic circuit
Coil of wire
Switch
S
Battery
+
-
Car sidelight bulb limits current
12V 5W
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Inductance of coil
The inductance of an electromagnet is
L (in Henrys) = n2  a / l
where:
n = no of turns
 = absolute permeability of core
a = area of coil in sq metres
l = length of coil in metres
The absolute permeability of air 0 is 4 X 107
Relative permeability r ranges from 1 for air, wood,
aluminium & plastics to 3000 for soft iron, silicon steel and
ferrites. Absolute permeability is just 0 * r
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Permeability 
• Permeability is a magnetic property of a material, and is
often expressed as 0 x r , where
 0 is a physical constant equal to exactly 4*pi*10-7 Henries /
meter and
 r is the relative permeability. r is equal to 1.0 for free space.
• Relative permeability r refers to a material's ability to
attract and conduct magnetic lines of flux. The more
conductive a material is to magnetic fields, the higher its
permeability.
• Most materials, including copper aluminium and gold
have r near 1.0. The metals that are notable exceptions
are nickel, cobalt, manganese, chromium and iron.
These are called ferro-magnetic materials, and can have
permeabilities as high as 100 or more.
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The strength of the magnetic field can be
increased by putting an iron rod inside the coil.
The shorter you make the air path the stronger the
field will be. (This is why horseshoe, button and
pot magnets are more popular than bar magnets.)
The permeability of iron is about 2000 times higher
than that for air, so the more iron and the less air
there is in the path the better.
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Permeability of common materials
Cobalt
68 – 245
Nickel
1240
Copper
1.0 – 1.2
Austenitic stainless steel
1.02 max
Martensitic stainless steel
900
Cast iron
100 – 750
Mild steel
380
Alloy 49 Iron-Nickel High Permeability Alloy
150,000
Special Metals NILOMAG™ Alloy 77
60,000 - 300,000
Supermalloy
1,000,000
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Horseshoe electromagnet
•To keep the air gaps small the coils can be
wound directly onto iron rods. (e.g. nails or
bolts) (It’s best to put a layer of paper first to
protect the insulation)
Short air path
for field
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Relay, contactor, or buzzer
The electromagnet can be used to operate a switch with
one or more sets of contacts – this is called a relay
Contacts
normally open normally closed
Armature
Pivot and
common contact
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Vibration annunciator
Here the iron slug slides freely inside the coil. When a current flows
through the coil it pulls the slug into the coil. The contact is broken,
and the current stops. The spring pulls the slug out again and the
process repeats. The frequency of vibration is determined mainly
by the strength of the spring and mass of the slug.
Coil of wire
Iron slug
Spring
+
Battery
-
Sliding
contact
12V 5W
Car sidelight bulb limits current
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Motor
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DC Machines (Motors & Generators)
• Permanent magnet
• Series coil
• Shunt coil
DC motors have the same construction as
DC generators. Apply a current and the
shaft will rotate; rotate the shaft and you will
generate a current.
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Generator DC
split ring
commutator
brush
field
V
The coil (which may be
many turns) is usually
wound onto a soft iron core
time
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Generators DC (other types)
Series
+-
Shunt
Compound
N
N
A
A
S
S
+ iV
N
A
S
+ i  or i 
V is const
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AC Generator (Alternator)
Electro
magnet
N
commutators
S
brush
wire
coils
AC output
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Transformer
A device for changing AC voltages
Vout = Vin * Ns / Np
Vin
Vout
primary
secondary
Np =6
Ns =12
soft iron laminated core
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Transformers
• Can step ac voltages
up and down
• Provide electrical
isolation between
circuits to protect
patient safety etc.
• Don’t work with dc
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Transformer example
A mains transformer provides 12 V ac for a 48W halogen
light. The voltage ratio needed is 240 V / 12 V so the turns
ratio needed is 20 (primary) : 1 (secondary) In practice
the transformer would have many more turns than this to
provide the right ratio.
Note that as the voltage is stepped down (240 V : 12V) so
the current is stepped up (0.2A to 4 A)
L
48W = 240V * 0.2A
48W = 12V * 4A
N
Iron cored transformer
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Resources
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www.gcsescience.com basic electromagnetism etc.
www.magnetsinfo.com/ bumf about metal and ceramic magnets
www.4qd.co.uk/ a thorough introduction to motors of all types
www.psigate.ac.uk Physics gateway
scienceworld.wolfram.com
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